Since non-line-of-sight (or indirect wave) communication may undergo serious multipath fading, an Automatic Repeat Request (ARQ) scheme is generally provided in a Medium Access Control (MAC) layer to achieve reliability of the communication. Advanced Antenna System (AAS) technologies are also provided to increase the cell coverage and the capacity of the system through beam forming using multiple antennas. The system may also support a Dynamic Frequency Selection (DFS) function to overcome coexistence problems with other systems in unlicensed bands.
FIG. 1 illustrates an example of a mesh-structured communication network. Generally, in broadband wireless access, it is possible to perform communication using not only a Point-to-Multipoint (PMP) structure but also the mesh structure as shown in FIG. 1. The mesh mode allows access to a base station through relay of another subscriber end, thereby actively coping with non-line-of-sight communication environments in cities where radio-wave shadow areas are present due to large buildings or the like.
FIG. 2 illustrates an example of a frame structure in the mesh mode. As shown in FIG. 2, the mesh mode may construct control subframes and data subframes instead of using conventional frames. Control subframes provide the following basic functions. A combination of different systems is created and maintained with network control subframes. A function for equivalent scheduling in data communication between systems is performed with schedule control frames. All frames other than network control subframes that are generated at regular intervals are schedule control frames.
FIG. 3 is a flow chart of an example of a network access process of a mobile station in the case of PMP. As shown in FIG. 3, first, when a mobile station (MS) is powered on, the MS searches for a down channel and obtains up/down synchronization with a base station (BS) (S31). The MS then performs ranging with the BS to adjust uplink transmission parameters and the BS then allocates a basic management Connection ID (CID) and a primary management CID to the MS (S32). The MS and the BS perform negotiation of basic performance (S33) and perform an authentication process (S34). When the MS has been registered in the BS, the MS establishes an IP connection after the BS allocates a secondary management CID to the MS which is managed with an IP (S35). The MS then sets current date and time (S36) and downloads an MS configuration file from a server (S37) and then establishes a service connection (S38).
The following is a description of a procedure in which an MS registered in the network performs preprocessing procedures for handover. The preprocessing procedures for handover include a network topology advertisement procedure in which a serving BS broadcasts information regarding neighbor BSs to notify MSs of the information, a procedure for MSS scanning of neighbor BSs in which channel qualities of neighbor BSs are measured based on the information, and an association procedure in which the MS selectively performs, in advance, a ranging process with neighboring BSs, thereby allowing the MS to adjust uplink transmission power values, time and frequency difference values for synchronization, and the like, and to adjust parameters associated with uplink transmission, and to obtain, in advance, basic management CIDs, primary management CIDs, and the like from the BS.
Through the network topology advertisement procedure as described above, the BS transmits information regarding network configurations in the form broadcasting a BS advertisement message (MOB_NBR-ADV) to all MSs in the cell. Especially, the serving BS transmits, in advance, up and down channel parameters of neighbor BSs, thereby allowing the MS to rapidly obtain synchronization with a BS when performing handover to the BS and thus to reduce the time required to perform handover. The channel parameters include an Uplink Channel Descriptor (UCD) and a Downlink Channel Descriptor (DCD).
The MS must perform handover as needed. Thus, it is necessary to perform a process for scanning neighbor BSs. For scanning neighbor BSs, the MS transmits a scanning request message (MOB_SCN-REQ) to receive a scanning interval allocated by the BS. Through a scanning response message (MOB_SCN-RSP), the BS allocates an interval in which the MS can search for neighbor BSs. The BS may transmit a (unsolicited) scanning response message (MOB_SCN-RSP) to the MS without request of the MS. Here, the BS may allocate the scanning interval and scanning start offset on a frame-by-frame basis.
In the association procedure, the MS performs, in advance, a ranging process with neighbor BSs as described above, thereby reducing the time required to perform handover. The ranging process is a process in which the MS obtains basic information for communication from the BS. The MS searches for neighbor BSs and selects an association target BS based on channel quality measurement results from the searched neighbor BSs. The MS then performs a ranging process with the BS to perform an association procedure.
In the association procedure, first, the MS transmits a ranging request message (RNG-REQ) or a Code Division Multiple Access (CDMA) code for ranging request. The BS sets power and timing offsets of the MS through a ranging response message (RNG-RSP). The BS also allocates a management CID to the MS through the ranging response message. After performing the association procedure with a specific MS, the BS stores association information of the MS.
The physical layer of the broadband wireless access system is classified into a single carrier type and a multicarrier type (OFDM/OFDMA). The multicarrier type uses Orthogonal Frequency Division Multiplexing (OFDM) and also uses Orthogonal Frequency Division Multiple Access (OFDMA) as an access method in which resources can be allocated in units of subscribers grouped using a part of the carrier.
FIG. 4 illustrates the concept of subchannels in the OFDMA physical layer Characteristics of the OFDMA physical layer in the broadband wireless access system are described as follows with reference to FIG. 4. In the OFDMA physical layer, an active carrier is separated into groups, which are transmitted to different receiving ends. The groups of the carrier for transmission to receiving ends are referred to as subchannels. FIG. 4 illustrates three subchannels including two subcarriers. Here, carriers included in each subchannel may be adjacent to each other or spaced at regular intervals. Allowing multiple accesses on a subchannel basis in this manner has advantages in that it is possible to efficiently perform frequency diversity gain control, gain control according to focusing of power, and forward power control.
FIG. 5 illustrates an example of a method for allocating resources in OFDMA. As shown in FIG. 5, a slot allocated to each MS is defined by a data region in two-dimensional space, which is a set of consecutive subchannels allocated by a burst. In FIG. 5, a data region in OFDMA is illustrated as a rectangle that is defined by a two-dimensional combination of the time and subchannel domains. The data region can be allocated to an uplink of a specific MS and information can be transmitted in downlink to the specific MS through the data region. The number of OFDM symbols in the time domain and the number of consecutive subchannels starting at a position that is spaced an offset from a reference point in the frequency domain must be given to define such data regions in two-dimensional space.
FIG. 6 illustrates an example of a method for mapping subchannels in uplink/downlink frames. As shown in FIG. 6, allocated subchannel areas are represented in two dimensions and data is mapped to the two-dimensional subchannel areas, starting from a subchannel of the foremost symbol. In the case of uplink, allocation areas of the allocated subchannel are first determined in one dimension. Specifically, a duration is determined and subchannel allocation is performed along the symbol axis, starting from a subchannel next to a subchannel that is previously allocated to another Protocol Data Unit (PDU) burst. When the last symbol of a specific subchannel is reached, the allocation is continued from the next subchannel.
FIG. 7 illustrates an example of a frame structure of the OFDMA physical layer of the broadband wireless access system. As shown in FIG. 7, a downlink subframe starts with a preamble used for synchronization and equalization in the physical layer. The overall frame structure is defined through downlink map (DL-MAP) and uplink map (UL-MAP) messages of broadcast format which define positions and usages of bursts allocated to the downlink and uplink.
Table 1 illustrates an example of the downlink map (DL-MAP) message.
TABLE 1SyntaxSizeNotesDL-MAP_Message_Format( ){Management Message Type8 bits= 2PHY Synchronization FieldvariableSee appropriate PHYspecification.DCD Count8 bitsBase Station ID48 bits Begin PHY Specific Section {See applicable PHYsection.for(i= 1; I <= n;i++) {For each DL-MAPelement 1 to n.DL-MAP_IE( )variableSee corresponding PHYspecification.}}if !(byte boundary) {Padding Nibble4 bitsPadding to reach byteboundary.}}
Table 2 illustrates an example of the downlink map (DL-MAP) message.
TABLE 2SyntaxSizeNotesUL-MAP_Message_Format( ) {Management Message Type =8 bits3UCD Count8 bitsAllocation Start Time32 bits Begin PHY Specific Section {See applicable PHYsection.for(i= 1; I <= n; i++) {For each UL-MAPelement 1 to n.UL-MAP_IE( )variableSee correspondingPHY specification.}}if !(byte boundary) {Padding Nibble4 bitsPadding to reach byteboundary.}}
As described above, the DL-MAP message defines usages of bursts allocated to a downlink interval in the burst mode physical layer and the UL-MAP message defines the usages of bursts allocated to an uplink interval.
Table 3 illustrates an example of information elements (IEs) that constitute the DL-MAP.
TABLE 3SyntaxSizeNotesDL-MAP_IE( ) {DIUC4 bitsif (DIUC == 15) {Extended DIUCvariableSee clauses following 8.4.5.3.1dependent IE} else {if (INC_CID == 1) {The DL-MAP starts withINC_CID = 0. INC_CID istoggled between 0 and 1 by theCID-SWITCH_IE( ) (8.4.5.3.7)N_CID8 bitsNumber of CIDs assigned for thisIEfor (n=0; n< N_CID;n++){CID16 bits }}OFDMA Symbol8 bitsoffsetSubchannel offset6 bitsBoosting3 bits000: normal (not boosted); 001: +6dB; 010: −6 dB; 011: +9 dB;100: +3 dB; 101: −3 dB; 110: −9dB; 111: −12 dB;No. OFDMA Symbols7 bitsNo. Subchannels6 bitsRepetition Coding2 bits0b00 - No repetition coding 0b01 -IndicationRepetition coding of 2 used 0b10 -Repetition coding of 4 used 0b11 -Repetition coding of 6 used}}
As in the example of Table 3, IEs, which constitute the DL-MAP, include a Downlink Interval Usage Code (DIUC), a Connection ID (CID), and burst position information used to identify downlink traffic intervals for MSs. The burst position information includes a subchannel offset, a symbol offset, the number of subchannels, and the number of symbols.
Table 4 illustrates IEs that constitute the UL-MAP.
TABLE 4SyntaxSizeNotes UL-MAP_IE( ) { CID16 bits  UIUC4 bits if (UIUC == 12) { OFDMA Symbol offset8 bits Subchannel offset7 bits No. OFDMA Symbols7 bits No. Subchannels7 bits Ranging Method2 bits0b00 - Initial Ranging/HandoverRanging over two symbols0b01 - Initial Ranging/HandoverRanging over four symbols0b10 - BW Request/PeriodicRanging over one symbol0b11 - BW Request/PeriodicRanging over three symbols reserved1 bit Shall be set to zero } else if (UIUC == 14) { CDMA_Allocation_IE( )32 bits  else if (UIUC == 15) { Extended UIUCvariableSee clauses following 8.4.5.4.3dependent IE } else { Duration10 bits In OFDMA slots (see 8.4.3.1) Repetition coding2 bits0b00 - No repetition codingindication0b01 - Repetition coding of 2used 0b10 - Repetition codingof 4 used 0b11 - Repetitioncoding of 6 used} Padding nibble, if4 bitsCompleting to nearest byte, shallneededbe set to 0. }
As in the example of Table 4, each information element (IE) of the UL-MAP message specifies the position of a corresponding interval by a duration field and specifies its usage by an Uplink Interval Usage Code (UIUC) for each Connection ID (CID). That is, the usages of the intervals of the UL-MAP message are defined by corresponding UIUCs used in the UL-MAP message. The interval of each UL-MAP IE starts at a position at a distance, corresponding to a duration defined in the UL-MAP IE, from the start of the previous IE. An uplink interval defined by “UIUC 12” is allocated to a usage for initial ranging, handover ranging, periodic ranging, or band request and has contention-based characteristics.
The following is a description of a ranging operation that is performed when an MS is initialized in a single carrier and OFDM method for the broadband wireless access system. A BS allocates a contention-based initial ranging interval to MSs through a UL-MAP message and each MS transmits a ranging request message including its MAC address using an uplink ranging interval. If the BS cannot decode a received ranging request message due to collision with another ranging request message transmitted from another MS, the BS transmits a ranging response message including uplink transmission parameter adjustment values and a ranging interval and a frame number that were used to receive the ranging request message.
If the BS can decode the received ranging request message, the BS transmits uplink transmission parameter adjustment values of the MS through a ranging response message. When adjustment of uplink transmission parameters of the MS is successfully performed, the BS transmits a ranging response message including a basic CID and a primary management CID to the MS. The BS allocates an uplink band through a UL-MAP message to allow the MS to transmit a ranging request message. Here, the BS allocates a non-contention-based uplink band through a basic management CID of the MS. When the uplink band has been allocated to the MS, the MS transmits a ranging request message and the BS transmits a ranging response message in response to the ranging request message. Here, the MS and the BS can perform their downlink burst coding and modulation method adjustment.
The following is a description of a ranging operation that is performed when an MS is initialized in an OFDMA method for the broadband wireless access system. In the OFDMA method for the broadband wireless access system, the MS can use a CDMA code for ranging request and uplink band request for adjustment of uplink transmission parameters. The BS transmits a set of CDMA codes for ranging and band requests to MSs through a UCD message in broadcast format. Each MS selects a ranging code suitable for the usage from the CDMA codes obtained from the UCD message and transmits it to the BS through an uplink interval allocated for ranging.
Table 5 illustrates an example of the UCD message.
TABLE 5SyntaxSizeNotesUCD_Message_Format( ) {Management Message Type = 08 bitsConfiguration Change Count8 bitsRanging Backoff Start8 bitsRanging Backoff End8 bitsRequest Backoff Start8 bitsRequest Backoff End8 bitsTLV Encoded information forvariableTLV specificthe overall channelBegin PHY Specific Section {See applicable PHYsection.for(i= 1; i <= n; i++) {For each uplink burstprofile 1 to n.Uplink_Burst_ProfilevariablePHY specific}}}
Table 6 illustrates an example of a TLV parameter associated with ranging and band requests included in the UCD message.
TABLE 6TypeName(1 byte)LengthValueInitial1501Number of initial ranging CDMA codes.rangingPossible values are 0-255.acodesPeriodic1511Number of periodic ranging CDMA codes.rangingPossible values are 0-255.acodesHandover?1Number of handover ranging CDMArangingcodes. Possible values are 0-255.acodesBandwidth1521Number of bandwidth request codes.requestPossible values are 0-255.acodesPeriodic1531Initial backoff window size for periodicrangingranging contention, expressed as a powerbackoffof 2. Range: 0-15 (the highest order bitsstartshall be unused and set to 0).Periodic1541Final backoff window size for periodicrangingranging contention, expressed as a powerbackoffof 2. Range: 0-15 (the highest order bitsendshall be unused and set to 0).Start of1551Indicates the starting number, S, of theranginggroup of codes used for this uplink. Allcodesthe ranging codes used on this uplink willgroupbe between S and ((S + N + M + L + O)mod 256). Where, N is the number ofinitial-ranging codes. M is the number ofperiodic-ranging codes. L is the number ofbandwidth-request codes. O is the numberof initial-ranging codes. M is the numberof handover-ranging codes. The range ofvalues is 0 S ≦≦ 255
The BS allocates a contention-based ranging interval to MSs through an uplink map IE included in a UL-MAP message. Here, the contention-based ranging interval may be allocated while being divided into an initial ranging and handover ranging interval and a periodic ranging and band request interval according to the ranging usages.
When receiving a ranging code from the MS, the BS sets and transmits a ranging state (success or fail), adjusted time and frequency values, an adjusted transmission power value required for uplink transmission synchronization of the MS, and the like through a ranging response message (RNG-RSP).
Table 7 illustrates an example of the ranging response message (RNG-RSP).
TABLE 7SyntaxSizeNotesRNG-RSP_Message_Format( ) {Management Message Type = 58 bitsUplink Channel ID8 bitsTLV Encoded InformationvariableTLV specific}
Table 8 illustrates an example of a TLV parameter included in a ranging response message.
TABLE 8TypeName(1 byte)LengthValue(variable-length)Timing14Tx timing offset adjustment (signed 32-Adjustbit). The time required to advance SStransmission so frames arrive at theexpected time instance at the BS. Unitsare PHY specific (see 10.3).Power21Tx Power offset adjustment (signed 8-bit,Level0.25 dB units)AdjustSpecifies the relative change intransmission power level that the SS is tomake in order that transmissions arrive atthe BS at the desired power. Whensubchannelization is employed, Thesubscriber shall interpret the power offsetadjustment as a required change to thetransmitted power density.Offset34Tx frequency offset adjustment (signedFre-32-bit, Hz units)quencySpecifies the relative change inAdjusttransmission frequency that the SS is tomake in order to better match the BS.(This is fine-frequency adjustmentwithin a channel, not reassignment to adifferent channel.)Ranging41Used to indicate whether uplink messagesStatusare received within acceptable limits byBS.1 = continue, 2 = abort, 3 = success,4 = rerangeRanging1504Bits 31:22 - Used to indicate thecodeOFDM time symbol reference that wasattri-used to transmit the ranging code.butesBits 21:16 - Used to indicate the OFDMAsubchannel reference that was used totransmit the ranging code.Bits 15:8 - Used to indicate the rangingcode index that was sent by the SS.Bits 7:0 - The 8 least significant bits ofthe frame number of the OFDMA framewhere the SS sent the ranging code.
Table 9 illustrates an example of a TLV parameter included in a ranging response message.
TABLE 9TypeName(1 byte)LengthValue (variable-length)Downlink72This parameter is sent in response toOperationalthe RNG-REQ Requested DownlinkBurstBurst Profile parameter.ProfileByte 0: Specifies the least robustDIUC that may be used by the BSfor transmissions to the SS.Byte 1: Configuration ChangeCount value of DCD defining theburst profile associated with DIUC.SS MAC86SS MAC Address in MAC-48AddressformatBasic CID92Basic CID assigned by BS at initialaccess.Primary102Primary Management CID assignedManagementby BS at initial access.CID
When it is determined that the uplink transmission parameter adjustment has been completed, the BS sets the ranging state to “success” and allocates an uplink band to the corresponding MS through an uplink IE (CDMA_Allocation_IE) to allow transmission of a ranging request message.
Table 10 illustrates an example of an uplink map IE used for allocating an uplink interval to the MS whose ranging is successful.
TABLE 10SyntaxSizeNotesCDMA_Allocation_IE( ) {Duration6 bitsRepetition Coding2 bits0b00 - No repetition codingIndication0b01 - Repetition coding of2 used 0b10 - Repetitioncoding of 4 used 0b11 -Repetition coding of 6 usedRanging Code8 bitsRanging Symbol8 bitsRanging subchannel7 bitsBW request mandatory1 bit 1 = yes, 0 = no}
When an uplink interval has been allocated to the MS, the MS transmits a ranging request (RNG-REQ) message including its ID (MAC address) to the BS. When receiving the ranging request message, the BS transmits a ranging response (RNG-RSP) message including a basic management CID and a primary management CID to the MS.
Table 11 illustrates an example of the ranging request message.
TABLE 11SyntaxSizeNotesRNG-REQ_Message_Format( ) {Management Message Type = 48 bitsDownlink Channel ID8 bitsTLV Encoded InformationvariableTLV specific}
Table 12 illustrates an example of a TLV parameter included in a ranging request message.
TABLE 12TypeName(1 byte)LengthValue (variable-length)Requested11Bits 0-3: DIUC of the downlink burstDownlinkprofile requested by the SS forBurstdownlink traffic. Bits 4-7: 4 LSB ofProfileConfiguration Change Count value ofDCD defining the burst profileassociated with DIUC.SS MAC26The MAC address of the SSAddressRanging31A parameter indicating a potentialAnomalieserror condition detected by the SSduring the ranging process. Settingthe bit associated with a specificcondition indicates that the conditionexists at the SS.Bit #0 -SS already at maximumpower. Bit # 1- SS already atminimum power. Bit #2 - Sum ofcommanded timing adjustments is toolarge.AAS410 = SS can receive broadcastbroadcastmessages 1 = SS cannot receivecapabilitybroadcast messages
Table 13 illustrates an example of a TLV parameter included in a ranging response message.
TABLE 13TypeName(1 byte)LengthValue (variable-length)Downlink72This parameter is sent in response to theOper-RNG-REQ Requested Downlink BurstationalProfile parameter.BurstByte 0: Specifies the least robust DIUCProfilethat may be used by the BS fortransmissions to the SS.Byte 1: Configuration Change Count valueof DCD defining the burst profileassociated with DIUC.SS86SS MAC Address in MAC-48 formatMACAddressBasic92Basic CID assigned by BS at initialCIDaccess.Primary102Primary Management CID assigned by BSManage-at initial access.ment CID
As described above, the initial ranging is a process in which an MS transmits a ranging request to a BS through a contention-based uplink ranging interval when performing network registration and the BS provides a ranging response to the MS. When the MS performs ranging through a contention-based uplink interval, the MS may fail to receive a ranging response from the BS since it collides with another MS that performs ranging. If the MS fails to receive a ranging response message from the BS in a specific time after transmitting the ranging request message, the MS retransmits a ranging request to the BS.
The BS can allow a specific MS to perform non-contention-based ranging by allocating a ranging interval to the MS. When the MS performs handover to another BS while the MS is in normal operating state, the MS must perform ranging with the BS. Here, the serving BS may notify the handover target BS of the handover of the MS and the handover target BS may allocate a non-contention-based ranging interval which allows the MS to perform ranging when the MS actually performs handover.
Table 14 illustrates an example of an uplink map IE used to allocate a non-contention-based initial ranging interval.
TABLE 14SyntaxSizeNotesFast_ranging_IE {Extended UIUC4bits0x06Length4bitsLength = 0x0bHO ID indicator1bit0: MAC Address is present 1:HO ID is presentReserved3bitsShall be set to zeroif (HO ID indicator ==1) {HO ID8bitsReserved40bitsShall be set to zero} else {MAC address48bitsMS MAC address as providedon the RNG_REQ message oninitial system entry}UIUC4bitsUIUC ≠ 15. A four-bit code usedto define the type of uplinkaccess and the burst typeassociated with that access.if (UIUC == 12) {OFDMA Symbol8bitsoffsetSubchannel offset7bitsNo. OFDMA Symbols7bitsNo. Subchannels7bitsRanging Method2bits0b00 - Initial Ranging over twosymbols0b01 - Initial Ranging over foursymbols0b10 - BW Request/PeriodicRangingover one symbol0b11 - BW Request/PeriodicRanging over three symbolsReserved1bitShall be set to zero} else {Duration10bitsIn OFDMA slots (see 8.4.3.1)Repetition coding2bits0b00 - No repetition codingindication0b01 - Repetition coding of 2used0b10 - Repetition coding of 4used0b11 - Repetition coding of 6usedReserved20bitsShall be set to zero}}
The above processes can be performed more efficiently using an RS. That is, use of the RS can increase the coverage and throughput of the BS. However, uplink resources are wasted in a communication system including a mobile RS if MSs in the RS coverage individually perform handover.